Silicon is the most widely used material for substrates of integrated circuits and forms the basis of the modern semiconductor industry. The processing of dielectric materials, primarily silicon and doped silicon, using chemical or plasma etching techniques, is a mature technology, developed mainly for the microelectronics sector. These “conventional” processes produce exceptional results for digital CMOS circuitry. However, MEMS devices, which are also fabricated on silicon, are analog devices that require much tighter spatial control and depth control than the digital CMOS circuitry.
Conventional chemical or plasma etching techniques lack the spatial control needed for high performance MEMS devices. Although the conventional etch process starts on an area having a selected diameter, the effect of the etch process extends beyond the etch dimension beyond the desired area, leading to reduced control of the material removal process.
In the early prior art of laser ablation, lasers are used to provide a directed source of radiation whose deposited laser energy leads to the thermal heating of the substrate. However, there are many situations where heating is not desired and is, in fact, harmful. In these situations, such lasers may not be used. For example, long wavelength lasers, such as infrared lasers, which cut by heating a material substrate rather than by controlled photochemical ablation, are normally not desirable for etching since the etched region undergoes heating effects leading to uncontrolled melting.
Short pulse width infrared lasers exhibit some improvement in the control of the etch process as pulse width is reduced. For example, U.S. Pat. No. 5,656,186, Morrow et al. describes a laser with a pulse width of 100 fs to 1 ps at a 800 nm wavelength. See also, U.S. Pat. Nos. 7,560,658, 7,649,153 and 7,671,295. Now laser ablation using short pulses at long wavelength typically involves Ti:Sapphire (Ti:AlO3) lasers with pulses of 100 fs (0.1 ps) at a wavelength of 800 nm. The 100 fs pulse avoids phonon-phonon or electron-phonon coupling, which begins to occur at about 1.0 ps, but requires threshold intensities in excess of 1013 W/cm2 with a per pulse ablation depth of 300-1,000 nm. This per pulse ablation depth is greater than the thickness of many microcircuit layers, which is an uncontrolled event for microcircuit processing.
Alternatively, long pulse width, short wavelength lasers etch materials efficiently, but the etch process is still not adequately controlled. See, e.g., U.S. Pat. Nos. 4,925,523 and 7,469,831. The laser deposits energy in a layer close to the surface of the material to be etched. A molten area forms leading to vaporization of the surface. The vapor pressure of the material aids removing the material by expulsion. Strong shock waves of the expulsion can lead to splatter, casting of material, and thermal cracking of the substrate, which interferes with the clean removal of the material.
This laser ablation using long pulses at short wavelength typically involves UV KrF excimer lasers, or similar ultraviolet lasers, with pulses of 1.0 ns or longer at a wavelength of 248 nm. However, this technique produces uncontrolled ablation with spalling and cratering. The uncontrolled ablation is a result of heating and melting of the material to be ablated beyond the laser spot size due to thermal (phonon-phonon) coupling during the laser pulse.
Thus, current technologies for laser ablation of materials use either long pulses at short wavelength or short pulses at long wavelength. Both technologies have significant short comings as described above. It is, therefore, desirable to ablate materials using lasers that have a short pulse length at a short wavelength. Such lasers remove material without undue heating or damage to the areas surrounding the laser sport and have the depth control desired. The ablation mechanism is different than that of the prior art.
Thus, a method and apparatus for controlled laser ablation of material is needed that avoids the spalling from longer pulses (>1.0 ps) and the excessive ablation from longer wavelengths. It is a goal of the present invention to achieve such controlled ablation through lowering the threshold intensity required for ablation in materials such as silicon; materials with crystalline substrates similar to silicon, defined herein as silicon nitride, silicon oxide, gallium arsenide, indium phosphide or sapphire; metals; or metal oxides.